1. Field of the Invention
The present invention relates, in general, to a method for preparing a perpendicularly magnetizable material usable on magnetic recording media and, more particularly, to a method employing a citric acid sol-gel technique without polyhydroxy alcohol to prepare Sr-ferrite simply, thereby improving the material in particle size and coercive force.
2. Description of the Conventional Art
Sr-ferrite, known as a material for permanent magnets, is utilized as a perpendicularly magnetizable material on magnetic recording media like Ba-ferrite. As magnetic media are required to possess higher data storage density, the magnetic material is required to have a higher coercive force.
The magnetic properties of a ferrite material are largely changed depending on its particle size, particle size distribution, shape, purity, and size and distribution of pores on its surface.
In order to be utilizable as a perpendicularly magnetizable material on high performance magnetic recording media, Sr-ferrite powder is required to be fine in size and uniformly distributed and to have a particle shape of a hexagonal plate. The reason for this is that as the particles become more fine and uniformly distributed, single domain orientation is more strongly induced, and that as they become closer in shape to hexagon, the shape anisotropy increases, inducing a higher coercive force.
In consideration of these factors, various techniques have been developed for the preparation of ferrite powder and are prevalent. Of these techniques, representative ones include a solid state synthesis, a coprecipitation technique, a hydrothermal synthesis, a sol-gel technique and glass-crystallization.
Solid state synthesis comprises the step of sintering a starting oxide at high temperatures for a long period of time to obtain a single phase of ferrite. This step, however, may cause the abnorminal grain growth (2 to 3 .mu.m), making the attainment of single domain orientation difficult. In addition, incorporation of impurities is likely to occur during a mixing step in the solid state synthesis, having a detrimental effect on its magnetic properties.
To overcome the disadvantages encountered in the solid state synthesis technique, a variety of wet synthesis techniques have been developed and include, as mentioned above, coprecipitation, hydrothermal synthesis, and alkoxide sol-gel synthesis.
The coprecipitation technique is mainly hydroxide and oxalate method. When precursors are synthesized by the coprecipitation technique, it is important to obtain a stoichiometric compound. In particular, in the case of hydroxide and oxalate method, a stoichiometric compound is difficult to obtain due to the large solubility difference between Sr(OH).sub.2 and Fe(OH).sub.3, and SRC.sub.2 O.sub.4 and Fe.sub.2 (C.sub.2 O.sub.4).sub.3 in an aqueous solution.
In addition, since aggregation of particles is unavoidable during the drying and calcination of coprecipitated precursor, it is difficult to make the particles fine, which subsequently degrades the coercive force.
With regard to the hydrothermal synthesis technique, a precipitate is initially formed as in the above mentioned hydroxide coprecipitation technique and then heated under pressure to yield a powder. Accordingly, the magnetic particles are improved in dispersibility by the activation of particle surface in the hydrothermal synthesis. However, in the hydrothermal synthesis technique, it is difficult to obtain a quantitative precipitation, so that extreme fluctuation in composition may occur. This, in turn, results in a problem that differences in coercive force may occur in accordance with the synthesis conditions.
An alkoxide technique, which utilizes the hydrolytic decomposition of metal-alkoxide, is effective for forming fine particles. But, the metal-alkoxide materials themselves are very expensive and difficult to handle due to their high hygroscopicity. In addition, in order to obtain a quantitative oxide, the hydrolysis conditions such as pH, temperature and concentration of a solution must be controlled precisely, but no general process for controlling the hydrolysis rate of the metal-alkoxide compound has been developed yet.
As another wet method, there is adopted a glass-crystallization technique. In this technique, a mixture of SrO and Fe.sub.2 O.sub.3 is well blended with a proper flux, such as B.sub.2 O.sub.3 or SiO.sub.2 and the substance is melted. In this technique, a high temperature, for example, above 1,300.degree. C., is required, and additional processes are necessary for removing the flux from the final products. As a result, the glass-crystallization technique has the problems that the production cost is high and the procedure is complicated.
U.S. Pat. No. 3,330,697 discloses a citric acid sol-gel technique which produces a fine powder by mixing citric acid with a metallic ion solution and adding to this mixture ethylene glycol to be used as a chain-forming agent to induce polymerization. This technique is advantageous in that the starting substance is relatively inexpensive and the synthetic procedure is relatively simple.